Perfusion Parameters
How are parameters like BF, BV, and MTT defined and used in perfusion imaging?
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The simplest model of regional circulation contains a single arterial input, a tissue capillary bed, and a single venous output. This model has a long history, originally used by Fick in the 19th Century to study cardiac output. Later modifications for indicator dilution blood flow measurements were made by Kety and Schmidt in the 1940s and extended by Meier and Zierler in the 1950's.
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Simple model of regional circulation with a single arterial input and single venous output. Blood flow (BF) and Blood Volume (BV) are assumed to be constant.
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Blood flow (BF) through the system and blood volume (BV) within the tissue are both assumed to be constant. The units for BF and BV are usually normalized by dividing their absolute values by the mass of tissue perfused, resulting in units of the form mL/min/100g and mL/100g respectively. No pooling or loss of blood is allowed, so that whatever flows in eventually flows out.
Note that some paths through the tissue are relatively straight while others are highly branched. This means that each red blood cell or contrast agent molecule takes a different amount of time to traverse the capillary bed (although all will eventually appear in the venous efflux). The mean transit time (MTT), measured in sec (or min), reflects the average time an injected tracer particle following the flow of blood would reside within the system.
Blood flow (BF), blood volume (BV), and mean transit time (MTT) are related by the central volume theorem, so that if two are known, the third can be computed. The connecting relationship is given by the equation
BF = BV / MTT
which can be easily remembered by considering units of measurement, i.e., that blood flow represents a volume of blood per unit time.
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In perfusion imaging studies, BF, BV, and MTT are commonly calculated and displayed as color-coded parameter maps. These provide complementary perfusion-related information that may be useful for clinical decision making.
For example, areas of cerebral infarction typically display decreased BV and BF, with increased MTT. Conversely, areas of ischemia without infarction (the 'penumbra') typically have reduced BF and increased MTT, but normal BV. In oncology BV is provides an index of capillary density and is often elevated in more malignant tumors. Conversely low BV in a treated but enlarging tumor may suggest radionecrosis rather than tumor progression. |
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Details of how these quantitative perfusion parameters are extracted from image data will be provided in the next several Q&A's.
References
Fick A. Ueber die Messung des Blutquantums in der Herzventrikeln. Sitz der Physik-Med Ges Wurzburg 1870; 2:16–28. (Classic paper where the "Fick Principle" is formulated").
Kety SS, Schmidt CF. The determination of cerebral blood flow in man by the use of nitrous oxide in low concentrations. Am J Physiol 1945; 143:53–66. (classic paper; worth a look)
Meier P, Zierler KL. On the theory of the indicator-dilution method for measurement of blood flow and volume. J Appl Physiol 1954; 6:731-744. (Lays out proof for the central volume theorem and derivation of mean transit time).
Zaharchuk G. Theoretical basis of hemodynamic MR imaging techniques to measure cerebral blood volume, cerebral blood flow, and permeability. AJNR Am J Neuroradiol 2007; 28:1850-8.
Fick A. Ueber die Messung des Blutquantums in der Herzventrikeln. Sitz der Physik-Med Ges Wurzburg 1870; 2:16–28. (Classic paper where the "Fick Principle" is formulated").
Kety SS, Schmidt CF. The determination of cerebral blood flow in man by the use of nitrous oxide in low concentrations. Am J Physiol 1945; 143:53–66. (classic paper; worth a look)
Meier P, Zierler KL. On the theory of the indicator-dilution method for measurement of blood flow and volume. J Appl Physiol 1954; 6:731-744. (Lays out proof for the central volume theorem and derivation of mean transit time).
Zaharchuk G. Theoretical basis of hemodynamic MR imaging techniques to measure cerebral blood volume, cerebral blood flow, and permeability. AJNR Am J Neuroradiol 2007; 28:1850-8.
Related Questions
How is the arterial input function used to extract more quantitative flow information from the DSC data?
How is perfusion defined and measured?
How is the arterial input function used to extract more quantitative flow information from the DSC data?
How is perfusion defined and measured?